PBX In-dialing

ABSTRACT

An arrangement is disclosed for providing direct in-dialing telephone service from a conventional, marker-controlled telephone office to the extensions of a PBX whose stations are served by a switch unit remote from the central office and controlled by a central control unit located in the central office. A circuit is provided at the central office which is called into operation on calls directed to such stations to associate the control unit instead of the conventional number group translator with central office marker to furnish the marker the identity of an idle trunk to the switch unit. Simultaneously, the control unit instructs the remote switch unit to establish a connection between the idle trunk and the station corresponding to the called number digits. In this manner the called number digits need not first be outpulsed to the switch unit by the central office and then returned to the control unit for translation.

United States Patent Richards Reissued Nov. 4, 1975 PBX IN-DIALING [75]Inventor: Philip Charles Richards, Geneva, Ill. ABSTRACT [73] Assignee:Bell Telephone Laboratories, An arrangement is disclosed for providingdirect in- Incorporated, Murray Hill, NJ, dialing telephone service froma conventional, mark- [22] Filed Nov 1 1971 er-controlled telephoneoffice to the extensions of a PBX whose stations are served by a switchunit remote [21] Appl. No.: 194,719 from the central office andcontrolled by a central control unit located in the central office. Acircuit is Related Patent Documents provided at the central office whichis called into op- Ressue of: eration on calls directed to such stationsto associate [64] Patent No.: 3,479,467 the control unit instead of theconventional number Issued: Nov. 18, 1969 group translator with centraloffice marker to furnish Appl. No.: 588,379 the marker the identity ofan idle trunk to the switch Filed: Oct. 21, 1966 unit. Simultaneously,the control unit instructs the remote switch unit to establish aconnection between [52] US. Cl. ..'179/ 18 ES; 179/18 AD the idle trunkand the station corresponding to the [51] Int. Cl. H04Q 3/54 callednumber digits. In this manner the called number [58] Field of Search179/18 AD, 18 ES, 18 B, digits need not first be outpulsed to the switchunit by 179/18 BA, 27 CA Primary ExaminerTh0mas W. Brown Attorney,Agent, or FirmH. R. Popper the central office and then returned to thecontrol unit for translation.

7 Claims, 3 Drawing Figures 309 L/NE mu/v/r ii L/IVK L/NK 305 FRAMEFRAME 300 lNCOMl/VG- 304 306 LLF- mu/vx LINE cc L TL F LL P 50/? 304SEA/DER 1-L//v/r CALL 307 J co m r/ua 30/ PROCESSOR $51) ER 303 SM 70com/1:32:22 2

N0. GR. No. GRR I 32/ T L "V5 327 EQUIPMENT CONTROL AND CALLED MFG/WANG3/9 NUMBER TPANSLATOR n 3// NUMBER TRANSLATOR +bm 5mm; 3/0 3/2 322, A00EQUIPMENT NUMBER REGISTER ca/vmoL AND CALLED N0. REGISTER SHIFT /-a 9-15/7-32 3/6 DAP 10 DAP /632 COUNTER c626 C625 w car Q I car 5m 1 cszaDAZXCO I CONTROL CG CGT 7 CtfiX/DAI 3/5 3/7 041x5 3 3 3/4 PBX IN-DIALINGMatter enclosed in heavy brackets appears in the original patent butforms no part of this reissue specification; matter printed in italicsindicates the additions made by reissue.

This invention relates to telephone switching systems and moreparticularly to the furnishing of inward dialing service to theextension stations of private branch exchanges.

A private branch exchange is a facility which permits both local callsamong stations having a community of interest as well as trunk callsthrough the central office. In recent years direct inward dialingservice has become available. In this type of service, calls from thecentral office to the stations of the private branch exchange may beautomatically switched through without the need of involving the PBXoperator in the call. Heretofore direct inward dialing service has beenachieved by causing the central office to transmit the called extensionstation number over a trunk incoming to the PBX to operate the PBXsinward switching train. Because central office PBX trunks, unlike mostother trunks, appear in the central office on the line side of theswitching network the modifications described in Burns et al. Patent3,264,415 were required to permit the central office to outpulse thecalled number digits on the line side of the switching network. Thesemodifications included the provision of a special line circuit for thecentral office PBX trunk which had link access to a dial pulse sender insomewhat similar fashion to that in which outgoing trunks are given linkaccess to outgoing senders.

While the foregoing arrangement has been quite satisfactory infurnishing-direct inward dialing service to conventional private branchexchanges, some shortcomings have been noted in employing this procedureto provide direct inward dialing service to the new electronic privatebranch exchanges which employ a plurality of remote switch unitsadministered by a centralized control unit located at the centraloffice. An electronic private branch exchange of this type is describedin Gebhardt et a1. Patent 3,225,144 issued Dec. 21, 1965. The remoteswitch units of this type of electronic private branch exchangeequipment establish a plurality of simultaneous telephone connectionsunder the control of the control unit which writes into the local memoryat each switch unit the instructions needed to close that units timedivision switches. The information required to be stored in each switchunits memory is received from the control unit over a respective datalink. The data link for each switch unit is also employed to transmitchanges in the supervisory states of the lines, trunks, and attendantkeys as detected by a local scanner at the switch unit. When a stationat one of the remote switch units dials or keys digits, the dialing orkeying condition is detected and an order is transmitted from thecentral office control unit over the data link to the switch unit toestablish a time division connection between the calling extensionstation and a digit trunk which runs between the switch unit and thecontrol unit. The digit trunk has access to a digit receiver at thecontrol unit which receives the called number and furnishes it to thecall processing equipment. The called number digits are translated bythe call processor equipment and orders are transmitted back to theswitch unit over the data link to instruct the switch unit to establishthe appropriate connections. If the call is an intraswitch unit call, atime division connection ismade between the calling and called stationsvia their appearances on the two different time division buses at theswitch unit. If the call is outgoing from the switch unit,the-information transmitted to the switch unit will contain aninstruction to seize an idle central office trunk appearing on the timedivision bus and establish a time division connection between thecalling subscriber and the central office trunk.

While the operation so far described is extremely efficient, thehandling of an incoming call is not quite so efficient because thecalled number pulsed from the central office over the trunk to theremote switch unit cannot be translated at the switch unit but must becontinued via a time division connection from the central office trunkto a digit trunk at the switch unit and then returned over the digittrunk to the digit receiver located in the control unit. This procedureresulted in the called number having to travel the distance out to theswitch unit and then back to the control unit. In addition, the need tomake a speech trunk to digit trunk connection at the switch unit reducedthe number of digit trunks available to the subscribers at the switchunit who might desire to use one of the trunks for dialing. It was alsopossible in this prior arrangement for the control unit to seize a trunkfor use on an outgoing call from the switch unit at the same time thatthe central office seized the same trunk to route an incoming call tothe switch unit. This double seizure possibility resulted in someimpairment of telephone service.

Accordingly, it is an object of the present invention to improve privatebranch exchange service and more particularly to improve direct inwarddialing service to private branch exchanges.

The improvement in in-dialing service to be accorded to stations ofremote switch units homing on a control unit located in the telephonecentral office is achieved by modifying the manner in which the centraloffice accomplishes the called number translation. The central officeselected for illustration is an office employing the No. 5 Crossbarapparatus of the type disclosed in A. J. Busch 2,585,904, issued Feb.19, 1952. This type of central office employs a marker which controlsthe establishment of cross-office linkages between line link and trunklink frames. To obtain the line link frame equipment locationcorresponding to a called directory number, the marker conventionallyobtains access to a number group translator. However, when the call isbeing made to a PBX, the number group, as an incident to providing theforegoing, extended sleeve leads to the marker from each of the PBXtrunks serving the called PBX. The marker tested the sleeve leads, and,together with the remainder of the information provided by the numbergroup, obtained the equipment location of an idle trunk to the PBX.

The use of the conventional number group, even when it was to furnishthe equipment locations of central office PBX trunks equipped for linelink pulsing so as to handle direct inward dialing calls, involvedcertain disadvantages. On each seizure by the marker the number group iscapable of extending only ten PBX trunk sleeve leads to the marker fortesting. If none of these trunks are available for use, the marker mustrecycle its terminal hunting equipment to test the sleeves of anadditional set of ten trunks serving the desired PBX. The need to resortto terminal hunting recycle introduces an undesirable delay into thesystem as well as adding to marker usage time. Moreover, the foregoingsomewhat cumbersome arrangement required an office equipped for linelink pulsing. If no such office was available in the immediate'vicinityof the PBX, the PBX trunk lines would have to be extended to a possiblymore remote office which was equipped for this function. It is apparenttherefore that the elimination of the need to adapt each PBX trunk forline link pulsing would achieve a considerable economy in central officeinstallation and at the same time speed up the handling of direct inwarddialing calls.

In accordance with the illustrative embodiment of the invention, directinward dialing service to stations of remote switch units homing on acontrol unit located in the central office is achieved by providing acentral office interface unit which permits the central office marker tocommunicate with the PBX control unit. The marker furnishes the calleddirectory number to the control unit. The control unit, independently ofthe marker, translates the directory number and ascertains the centraloffice equipment location of an idle trunk serving the correspondingswitch unit. The control unit then transmits to that switch unit, overthe appropriate data link, the identity of the trunk so selected as wellas the identity of the station corresponding to the called 7 directorynumber. The control unit also informs the marker of the equipmentlocation of the selected trunk and the marker establishes a cross-officeconnection to that trunk without performing any trunk hunting or otherfunctions priorly required when establishing connectionsto PBX trunks.Because the control unit maintains a record of available PBX trunks andis responsible for selecting trunks both on calls originated at theremote switch unit as well as for calls incoming to switch units, thereis no possibility of a PBX trunk being simultaneously seized for bothpurposes.

The foregoing and other objects and features may become more apparent byreferring to the drawing in which:

remote switch units may perhaps be best understood after considering themanner in which direct inward dialed calls have heretofore beencompleted. When the marker 301, FIG. 3, receives the directory number ofthe called telephone (originally transmitted to the central office overcalling trunk 300 appearing on trunk link frame TLF) the thousands digitenables the marker to select a number group 303 by operating acorresponding number group connector 302-1. The marker passes to theselected number group over parallel leads the hundreds, tens, and unitsdigits of the called directory number. In the number group, asdescribed, for example, in Busch Patent 2,585,904 issued Feb. 19, 1952,hundreds block, tens block, and units digit relays are operated by thisinformation to select a set of directory number code pointscorresponding to the called directory number. There are three codepoints (not shown) 4 foreach directory number and separate jumpers arerun from each code point through translation fields to provide theequipment location and ringing combination associated with thecallednumber.

If the called number is at a private branch exchange, there normallywill be a plurality of trunks such as trunk 304 between the centraloffice and the private branch exchange. In this case, besides the tensblock relay of the number group there will be operated, in parallel, anSC (sleeve connect) relay, not shown in the drawing, whose contactsprings have access up to ten of the trunks serving the PBX. When the SCrelay is operated, these sleeve leads are extended through the numbergroup to the marker. The number group furnishes the marker a silentringing combination also indicating a terminal hunting group. Uponreceipt of this indication, the marker tests the ten sleeve leads andtransmits a units code for the first idle trunk found. Where there aremore than ten trunks in a PBX group, SC relays will be associated withall the tens block relays that have trunks of that group, and the markermay advance from one to another if all trunks associated with one tensblock relay are busy. The marker, upon obtaining the location of an idlePBX trunk, such as trunk 304, places the trunk in the off-hookcondition.

It will be observed that thus far central office 'operation hasproceeded without regard to conditions at the PBX. Accordingly, it mayhappen that the PBX is attempting to seize the same trunk 304 for use ona trunk call at the same time that the central office marker determinedthe trunk to be available for its use. Under these circumstances aglare" condition may result in which the trunk is unavailable for usefrom either end.

With the trunk seized at the central office, outpulsing of the callednumber to the PBX could proceed if the PBX to which the trunk isconnected were of the conventional step by step type. However, theelectronic PBX disclosed in Patent 3,225,144 provides a switch unit101-2 for example, FIG. 1, at the remote end of the trunk which does notoperate autonomously. ()utpulsing is therefore not permitted until theswitch unit scanner 103 detects the off-hook condition of trunk 304 andso informs the control unit, FIG. 2. Although trunk 304 is connected toswitch unit 101-2, the following description will refer for simplicityof drawing only to the corresponding internal elements shown in detailfor switch unit 101-1, it being understood that such elements are alsopresent in switch unit 101-2. The switch unit l01-2 must then beinstructed to establish a connection from trunk 304 to one of digittrunks 210. The manner in which this takes place is as follows: Theswitch unit scanner 103 first detects the off-hook condition of PBXtrunk 304 and temporarily halts its scanning. Scanner 103 sends theoff-hook condition and the number of the PBX trunk at switch unit 101-1to data transmitter 104. Data transmitter 104 sends this message to thedata receiver 204 at the control unit, FIG. 2. Upon request of callprocessor 220, the message is passed from the data and digit store 207to the call processor. Upon receiving the trunk origination, callprocessor 220 chooses an idle time slot, an idle one of digit trunks 210and of digit receivers 212. The digit trunk number, PBX trunk equipmentlocation, time slot number, and a scanner start signal are passed to thedata and digit store 207 for transmission to the switch unit. The datatransmitter 215 at the control unit transmits this message to the datareceiver 105 at the switch unit. The data receiver 105 passes theincoming message to the data distributor 106. The data distributor 106writes the PBX trunks equipment number and the digit trunk number intothe proper time slot in the switch store 107, thus connecting PBX trunk304 to one of digit trunks 210 over the transmission bus. The datadistributor 106 also passes a scanner restart signal to scanner 103. Thecall processor 220 writes the digit trunk number and the digit receivernumber into the chosen time slot in the digit receiver connector 211connecting the selected digit trunk and digit receiver. The connectionthat now exists extends from the PBX trunk at the line link frame LLF(FIG. 3) of the central office to the remote switch unit 101-1, over thetransmission bus thereat and back to one of digit receivers 212 in thecontrol unit. The call processor 220 may now send a start signal throughthe trunk connector 218 of the trunk control section 219 back over thePBX trunk to line circuit 306 (FIG, 3). In accordance with theprocedures disclosed in Burns et al. Patent 3,264,415, issued Aug. 2,1966, a line link pulsing sender circuit 307 associated with linecircuit 306 of PBX trunk304 will have been furnished the station digitsof the called stations directory number and, responsive to the startsignal, will commence outpulsing the digits serially. The digits sooutpulsed are received at the switch unit, forwarded over thetransmission bus to digit trunk 210, and entered into digit receiver 212at the control unit. Digit control 213 samples the output of the digitreceiver 212 and directs that the dialed digits are to be placed in thatportion of data and digit store 207 associated with the digit receiverused. Upon request of the call processor 220, the digits and the digitreceiver number are passed forward from the data and digit store to thecall processor. Call processor 220 then translates the dialed digits toobtain the switch unit equipment location of the called station so thatringing may be applied thereto and, eventually, a talking connectionestablished.

In this prior art method of providing direct inward dialing to a calledstation at the switch unit it will be observed that the control unitcould not translate the directory number of the called station into itsswitch unit equipment number and could not establish a connection to thecalled station at the switch unit until the called stations directorynumber was outpulsed to the switch unit and then back over a digit trunkto the control unit. Not only is this a time-consuming and circuitousprocedure but it is also wasteful of digit trunk capacity, cutting downon the number of simultaneous calls capable of being originated bystations at the switch unit. In addition, since the marker conducts theselection of PBX trunks independently of the switch unit, it is possiblefor a PBX trunk simultaneously to be seized by the marker for a callincoming to a station at the switch unit at the same time that thecontrol unit is seizing the same trunk for a different call originatedby another station at the switch unit.

In accordance with the principles of the illustrative embodiment, thedirectory number information furnished marker 301 is employed by themarker to operate connector 302-2 instead of connector 302-1 wheneverthe marker determines from the directory number that a call is incomingover trunk 300 destined for a station at one of the remote switch units101-1, 102-2 (FIG. 1). This determination is accomplished from theoffice code and thousands digit and merely requires that the thousandtelephone numbers designated by the thousands digits be dedicated forassignment only to switch unit telephones. In accordance with an aspectof our invention, direct inward dialing to the remote switch units isobtained without modification of the central office operations sincemarker 301 treats the direct access interface circuit 310 in the samemanner as a standard number group translator 327, the one beingaccessible by connector 302-2 and the other by connector 302-1.

Let it be assumed that the call is for station at switch unit 101-1.When connector 302-2 is operated, parallel leads are extended frommarker 301 to direct access interface circuit 310. The thousands,hundreds, tens, and units digits of the called directory numberregistered in completing marker 301 are passed to control and callednumber translator 311. These four digits of the called directory numberare converted in translator 311 from one-out-of-n form to the binaryform employed by call processor 220. Translator 311 advantageously alsoincludes checking circuits to insure that one, and one only, of each ofthe one-out-of-n coded called number designations sent by the marker isreceived at the interface circuit. An even-odd parity bit is alsoprovided by translator 311 in well-known manner.

In addition to the digits of the called number, the marker furnishesinterface circuit 310 with an origination mark which indicates whetherthe call is from an incoming trunk, an operator toll position, or a testfacility. When all of the information has been translated and checked,the information is entered in register 312. Register 312 includes aplurality of binary gates which make the information available to beentered into call store output register 222 when called for by commandtranslator 225.

Two gates 313 and 314, respectively enabled by gating command translator225 outputs CGZ3 and C025, are provided to pass the information storedin register 312 to call store output register 222. During the CGZ3command, the information containing the results of the one-out-of-ncheck, the parity information and the origination mark are accepted byregister 222. After accumulating this information, the processingprogram being executed will instruct command translator 225 to furnishthe CGZS command. This command causes the binary-coded called number tobe accepted by register 222. The parity of the called number is comparedwith that previously registered during the execution of the CGZ3command. Let it be assumed that the parity checks properly. From thecalled number information, program control, by consulting with programstore 224, will determine at which switch unit the called station islocated. After this is determined an idle one of central office trunks305 that serves the called switch unit is selected. The identity of thistrunk and that of the called station number is entered into call store223.

Each such central office trunk also has its equipment location number,which identifies its appearance in central office line link frame LLF(FIG. 3), associated with it in program store 224. When this trunk isselected to service the called station, the centrl office line linkframe equipment location number is obtained frorr program store 224 andentered in call store output register 222 for transmission to interfacecircuit 310 Command translator 225 is instructed by the call pro cessingprogram to furnish the CGZ4 command. Thi: command activates gate 316 andcauses the vertica and horizontal group information part of the centralof fice trunks equipment location to be accepted by regis ter 318. Thecall processing program then instructs the 7 command translator tofurnish the CGZ6 command. On this command gate 317 is activated and theframe units, frame tens and. vertical file information part of thecentral office trunks equipment location, as well as the ringingcombination associated therewith, is accepted by register 318. Thebinary information so registered is applied to translator 319 forconversion from binary form to the one-out-of-n form acceptable tomarker 301.

When marker 301 receives the equipment location and ringing combinationinformation pertaining to the selected one of trunks 305, it completes across-office linkage thereto from incoming trunk 300 in the normalmanner. Marker 301 is not given any trunk hunting information and so thecross-office connection may be made in the same manner as would beappropriate for completing an incoming call to an ordinary telephone set309. The sole exception is in the ringing combination employed, i.e.,silent ringing. When the crossoffice connection is established, theswitch unit end of the selected trunk exhibits an off-hook condition.

At the same time as the foregoing, switch unit scanner 103 is routinelyscanning lines and trunks at the switch unit and furnishing datatransmitter 104 with scan point messages for the control unit. When thecentral office has seized the selected one of trunk circuits 305 inaccordance with the above description, this condition will be detectedby scanner 103 and a corresponding scan point message will be forwardedto the control unit. The control unit will read from call store 223 theprogress mark associated with the scan point message which correspondsto the selected trunk and determine therefrom that this trunk has beenappropriatelyseized by the central office. Call store 223 is thenchecked to determine if the called station is busy.

Assuming the called station to be idle, the time slot number, the calledstations equipment number, the number of the selected central officetrunk, and a ringing instruction are passed to data and digit store 207for transmission to the switch unit. Data transmitter 215 transmits thismessage to data receiver 105 at the switch unit where it is passed todata distributor 106. Data distributor 106 writes the called stationsequipment number, the number of the central office trunk,

and the ringing instruction into the switch store 107.

Ringing is applied at the switch unit and when the called station goesoff-hook, ringing is tripped at the line circuit. The scanner 103detects the off-hook condition of the called station and passes thisinformation together with the called stations equipment number to thedata transmitter 104 for transmission to the control unit. The datareceiver 204 at the control unit passes the message to the data anddigit store 207 which then forwards the information to the callprocessor 220 when requested. The call processor 220 compares themessage to existing records in the call store 223 and identifies themessage with the call being set up. The scanner start signal, the calledstations equipment number, the number of the central office trunk, andthe time slot number are passed to the data and digit store 207 fortransmission back to the switch unit. The data and digit store 207 sendsthe message to the data distributor 106 at the switch unit whichrestarts the scanher 103, and writes the called stations equipmentnumber and the trunk number into the switch store 107. A communicationsconnection is now established between the central office trunk and thecalled station. The call processor 220 then may send an off-hook sig- 8nal through the trunk connector 218 to the selected one of trunkcircuits 305 which indicates to the central office that the call hasbeen answered.

From the foregoing, it will be seen that by the use of direct accessinterface circuit 310, the line link pulsing line circuit 306, line linkpulsing sender 307, line link pulsing number group 303, and connector3021 may completely be eliminated. Elimination of this equipment notonly permits the comparatively simpler registers, gates, and binarytranslator equipments of direct access interface circuit 310 to beemployed instead, but

also materially speeds up call completion. With the use of direct accessinterface circuit 310, central office 4BX trunks will no longerencounter double seizure problems since all such trunks are assigned bycall pro cessor 220. The transmission of called number information ismade directly in the central office from the marker 301 throughinterface circuit 310 to call processor 220 instead of by the formercircuitous route by way of the switch unit. It should however be notedthat the use of the direct access interface circuit 310 is entirelycompatible with the continued use, side by side in the same centraloffice, of the apparatus 302-1, 303, 306, and 307 for line link pulsingto certain switch units such as 1012.

It will be apparent to those skilled in the art that certainmodifications may be made in the system as described herein. However, itis to be understood that the above descriptive arrangements are merelyillustrative of the principles of the invention and that otherarrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of this invention.

What is claimed is:

1. A telephone switching system comprising a central office having amarker-controlled switching network, a plurality of telephone stationsat a remote switch unit, a plurality of trunks connecting said switchunit with said central office switching network, a stored programcontrol unit for exchanging control information with said switch unit toestablish communications connections among said stations and trunksthereat, and translator means including said control unit for furnishingthe marker of said central office switching network with the [centraloffice equipment location] identifcation of an idle one of said trunksserving called telephones at said switch unit.

2. A telephone switching system according to claim 1 further includinginterface circuit means, said stored program control unit and saidmarker of said central office switching network being connected withsaid interface circuit means.

3. A telephone switching system according to claim 2 wherein said storedprogram control unit includes command translator means and wherein saidinterface circuit includes gating means operable under control of saidcommand translator means.

4. A telephone switching system according to claim 3 wherein saidinterface circuit includes means for receiving called telephone numbersfrom said marker of said switching network, means for translating saidnumbers into a code form acceptable to said stored program control unit,means for receiving said equipment location of said idle one of saidtrucks from said control unit and means for translating said equipmentlocation into a code form acceptable to said marker.

5. A telephone switching system according to claim 4 wherein said meansfor receiving includes means for checking that said called telephonenumbers received from said marker are in said form acceptable to saidstored program control unit and wherein said gating means includes afirst gate controlled by said command translator for transferring saidcalled telephone numbers translated by said translating means to saidstored program control and a second gate controlled by said commandtranslator for transferring said called telephone numbers to said storedprogram control.

6. In a telephone switching system a plurality of remote switch units, acentral office including an electronic control unit for said switchunits, data transmission means between said electronic control unit andeach of said switch units, a first and a second plurality of trunksconnected to said central office, said central office further comprisingnumber group translation means for identifying one of said trunks insaid central office to which said central office is to establish aconnection, marker means, means controlled by said marker means forobtaining from said number group translation means trunk identificationsof said first plurality of trunks, and means controlled by said markermeans for obtaining trunk identifications of said second plurality oftrunks from said electronic control unit, said control unit thereuponcontrolling one of said switch units over said data transmission meansto establish a connection to one of said second plurality of trunks atsaid switch unit and said marker controlling the establishment of aconnection to said one of said trunks at said central office.

10 7. A telephone central office having a marker controlled switchingnetwork, a plurality of telephone stations at each of a plurality ofremote switch units, a plurality of trunks connecting said switch unitswith said central office switching network, a stored program controlunit for exchanging control information with said switch unit toestablish communications connections among said stations and trunksthereat, means for connecting the marker of said switching network withsaid control unit when said switching network is processing a callincoming for one of said stations at one of said switch units, saidconnecting means including means for receiving from said marker thedirectory number of the called one of said stations, means forfurnishing said directory number to said control unit, means forreceiving from said control unit [the central office equipment location]identifcation of an idle one of said trunks serving the switch unithaving said called one of said stations, and means for furnishing [said]the equipment location of said idle one of said trunks to said marker,means at said remote switch unit for determining when said marker hasseized said idle one of said trunks responsive to said furnishing ofsaid equipment location, and means at said control unit for instructingsaid switch unit to interconnect said one of said trunks with saidcalled one of said stations.

1. A telephone switching system comprising a central office having a marker-controlled switching network, a plurality of telephone stations at a remote switch unit, a plurality of trunks connecting said switch unit with said central office switching network, a stored program control unit for exchanging control information with said switch unit to establish communications connections among said stations and trunks thereat, and translator means including said control unit for furnishing the marker of said central office switching network with the (central office equipment location) identification of an idle one of said trunks serving called telephones at said switch unit.
 2. A telephone switching system according to claim 1 further including interface circuit means, said stored program control unit and said marker of said central office switching network being connected with said interface circuit means.
 3. A telephone switching system according to claim 2 wherein said stored program control unit includes command translator means and wherein said interface circuit includes gating means operable under control of said command translator means.
 4. A telephone switching system according to claim 3 wherein said interface circuit includes means for receiving called telephone numbers from said marker of said switching network, means for translating said numbers into a code form acceptable to said stored program control unit, means for receiving said equipment location of said idle one of said trucks from said control unit and means for translating said equipment location into a code form acceptable to said marker.
 5. A telephone switching system according to claim 4 wherein said means for receiving includes means for checking that said called telephone numbers received from said marker are in said form acceptable to said stored program control unit and wherein said gating means includes a first gate controlled by said command translator for transferring said called telephone numbers translated by said translating means to said stored program control and a second gate controlled by said command translator for transferring said called telephone numbers to said stored program control.
 6. In a telephone switching system a plurality of remote switch units, a central office including an electronic control unit for said switch units, data transmission means between said electronic control unit and each of said switch units, a first and a second plurality of trunks connected to said central office, said central office further comprising number group translation means for identifying one of said trunks in said central office to which said central office is to establish a connection, marker means, means controlled by said marker means for obtaining from said number group translation means trunk identifications of said first plurality of trunks, and means controlled by said marker means for obtaining trunk identifications of said second plurality of trunks from said electronic control unit, said control unit thereupon controlling one of said switch units over said data transmission means to establish a connection to one of said second plurality of trunks at said switch unit and said marker controlling the establishment of a connection to said one of said trunks at said central office.
 7. A telephone central office having a marker controlled switching network, a plurality of telephone stations at each of a plurality of remote switch units, a plurality of trunks connecting said switch units with said central office switching network, a stored program control unit for exchanging control information with said switch unit to establish communications connections among said stations and trunks thereat, means for connecting the marker of said switching network with said control unit when said switching netwoRk is processing a call incoming for one of said stations at one of said switch units, said connecting means including means for receiving from said marker the directory number of the called one of said stations, means for furnishing said directory number to said control unit, means for receiving from said control unit (the central office equipment location) identification of an idle one of said trunks serving the switch unit having said called one of said stations, and means for furnishing (said) the equipment location of said idle one of said trunks to said marker, means at said remote switch unit for determining when said marker has seized said idle one of said trunks responsive to said furnishing of said equipment location, and means at said control unit for instructing said switch unit to interconnect said one of said trunks with said called one of said stations. 